The object of this Phase I proposal is to assess the accuracy of a non-invasive, isothermal rebreathing method for measuring the breath alcohol level compared to the subject's blood alcohol. Phase I will encompass the design and fabrication of a prototype device and its testing on human subjects. If successful, the design could lead to a low cost, portable device capable of gaining a significant share of the identified $224 million total worldwide market for alcohol breath testing units which give accurate results when used in patrol cars.